4 HDDs in an old computer


Normal BIOS in old 2- or 386 computers (sometimes even in early 486s) allows only 2 hard drives connected to one controller channel - IDE or MFM. However, it is possible to make this computer detect more hard drives by installing a second ISA controller card and running a DOS driver.

First, let's start with a small explanation about hard drives in 286 and 386 AT computers. XTs are much different, they have own BIOSes in controller boardss. In AT computers each HDD controller occupies some resources:

- Interrupt request: 14 (Primary) or 15 (Secondary in newer computers) (IDE), or none (MFM, older IDE)

- Memory port address: 1F0-1F7h / 3F6-3F7h (Pri), or 170-177h / 376-377h (Sec), (IDE, MFM)

Old computer's BIOS can only access first port address, which is a primary channel. 

Adding a new card.

Because controller's/computer's BIOS still handles primary channel, it's meaningless is it IDE or MFM. It could be even ESDI, but probably not SCSI, because in most SCSI controller cards its own ROM decides about drive handling and it may hack a secondary channel for more SCSI drives. The second controller must handle 170-177h address and no IRQ, so it will be probably an IDE controller. I've never seen MFM controllers with drive auto-detect routines needed by our driver, but theoretically it's possible to run two more MFM drives (by re-aligning controllers' BIOSes or using OnTrack DiskManager undocumented driver feature, look here: http://www.uncreativelabs.net/textfiles/#drives). So looking for an IDE controller which will be good as a secondary channel, you have to find one which has "master/slave" jumpers (they usually switch address, sometimes IRQ) or address selection jumpres and IRQ14 disabling jumper. However, it is theoretically possible to force controller card to not work in IRQ14 by disconnecting IRQ14 pin of ISA bus - for example by a piece of sticky tape, but I've never tried it!.

If you set up your card an plugged it in, it's time to install the hard drive(s): They must be already partitioned (the driver works with one big primary partition for one drive) and formatted using another computer with the same version of MS-DOS. Install the drive(s) in your PC as usual connecting them to the secondary card.

You won't see these drives in DOS or FDisk now. But let's test are they visible in computer's memory: Download this pack of tools:

4 HDD Utilities disk

And run WDTBLCHK.exe If your IDE drives aren't visible in DRIVE 3 and DRIVE 4 in one of tests, something is wrong.

Now installing the driver. Prepare 3Drives or 4Drives tool using patches. Copy 4_DRV300.HDD or 3_DRV.HDD (the first is for two additional drives, the second is for one, Secondary Master drive) to your root directory of MS-DOS disk and add this line to your CONFIG.SYS file:




Place it somewhere near the begining, maybe after initialization of memory drivers. Save and reboot your computer. You should see driver detecting additional drives.

The attached utility 4_PARK.EXE will park additional drives, if they need this.

MCbx, 2011

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